The basic budget problem is simple. For decades, the expansion of Social Security, Medicare and Medicaid — programs mostly for the elderly — was financed mainly by shrinking defense spending. In 1970, defense accounted for 42 percent of the federal budget; Social Security, Medicare and Medicaid were 20 percent. By 2008, the shares were reversed: defense, 21 percent; the big retirement programs, 43 percent. But defense stopped falling after Sept. 11, 2001, while aging baby boomers and uncontrolled health costs keep retirement spending rising.

Left alone, government would grow larger. From 1970 to 2009, federal spending averaged 20.7 percent of the economy (gross domestic product). By 2020, it could reach 25.2 percent of GDP and would still be expanding, reckons the Congressional Budget Office’s estimate of President Obama’s budgets. In 2020, the deficit (assuming a healthy economy with 5 percent unemployment) would be 5.6 percent of GDP. To cover that, taxes would have to rise almost 30 percent.

A VAT could not painlessly fill this void. Applied to all consumption spending — about 70 percent of GDP — the required VAT rate would equal about 8 percent. But the actual increase might be closer to 16 percent because there would be huge pressures to exempt groceries, rent and housing, health care, education and charitable groups. Together, they account for nearly half of $10 trillion of consumer spending. There would also be other upward (and more technical) pressures on the VAT rate.

Does anyone believe that Americans wouldn’t notice 16 percent price increases for cars, televisions, airfares, gasoline — and much more — even if phased in? As for a VAT’s claimed benefits (simplicity, promotion of investment), these depend mainly on a VAT replacing the present complex income tax that discriminates against investment. That’s unlikely because it would require implausibly steep VAT rates. Chances are we’d pay both the income tax and the VAT, making the overall tax system more complicated.